This invention relates to optical devices.
Wavelength-selective optical fibre devices such as optical fibre gratings are commonly used in optical communication links. An example of this is the use of chirped optical fibre gratings connected to an optical fibre link to provide compensation against the dispersion of the optical fibre link.
Chirped fibre gratings are particularly useful in this type of application, as they are compact, passive and relatively simple to fabricate. It has been proposed that the dispersion compensation given by this technique will allow currently installed step-index optical fibre links to be upgraded to higher bit rates at, for example a wavelength of 1.5 xcexcm (micrometers).
Chirped optical fibre gratings are inherently narrow-band devices, with a dispersion-bandwidth product proportional to the grating""s length. Two conflicting requirements then arise. Firstly, the dispersion of the grating must be sufficient to compensate for that of the fibre link, which in turn is generally proportional to the length of the fibre link. Secondly, however, the bandwidth of the grating must be sufficient no only for the optical bandwidth of the signal being transmitted via the fibre link, but also to allow for inaccurate specification or temporal drift of the optical transmitter""s centre wavelength.
The conflicting effect of these two requirements means that a grating 1 metre long would be required to provide a 5 nm (nonometre) bandwidth and a dispersion sufficient to compensate a 100 km (kilometre) link of currently standard telecommunications fibre. However, current technology does not provide a convenient technique for fabricating such a long grating, and gratings of about one tenth of this length are at the limit of present fabrication techniques.
It has been proposed that these problems can be avoided, if the grating is made to track the transmitter""s centre wavelength. This would allow a narrower bandwidth grating to be used, so increasing the dispersion available for a particular grating length. (The inverse of this proposal, where the transmitter is locked on to the grating, is undesirable in multiple-grating systems).
Previously proposed techniques for varying the wavelength response of a fibre grating include stretching and compressing the grating using a linear piezoelectric transducer (PZT), or mounting the grating on a cantilever member which is then bent by a linear PZT attached to the free end of the cantilever member. However, in the linear PZT technique the grating is prone to buckling, and in the cantilever technique the grating will tend to become chirped (or an existing chirp of the grating will undesirably vary) as the cantilever bends.
U.S. Pat. No. 4,703,287 discloses a phase modulator formed by mounting an optical fibre on a bimorph element.
This invention provides an optical device comprising an optical fibre grating having wavelength-dependent optical characteristics mounted on a bimorph element operable to bend in response to an electrical control signal, so that the wavelength-dependency of the optical characteristics of the optical fibre grating vary in response to bending of the bimorph element.
By using a bimorph element in this way, a uniform compression or stretch (not easily obtainable with the cantilever technique) can be applied to the optical fibre grating, thus varying its wavelength-dependent properties but without necessarily changing the grating""s chirp. The fibre can be securely fastened (e.g. glued) to the bimorph element along its length, avoiding the problems of fibre buckling.
Preferably the optical fibre grating is a chirped optical fibre grating. Preferably the grating is apodised so that the modulation of refractive index of the fibre has a substantially cosine-shaped envelope.
In order to increase the amount of compression or stretching applied to the fibre device, it is preferred that the bimorph element comprises more than two active layers of piezoelectric material. It is also preferred that the bimorph element comprises a plurality of active layers of piezoelectric material and an inert buffer layer disposed on the active layers, the optical fibre grating being attached to the buffer layer.
In order that the wavelength-dependent characteristics of the device can be made to track those of, say, an optical transmitter, it is preferred that the device comprises a feedback control circuit for detecting whether the wavelength-dependent characteristics of the optical fibre grating match those of a received optical signal, and, if not, for adjusting the electrical control signal so that the wavelength-dependent characteristics of the optical fibre grating more closely match those of the received optical signal.
In order to detect wavelength tracking errors and to determine an appropriate direction for applying a corrective signal, it is preferred that the device comprises means for applying a dither signal to the electrical control signal, and/or the means for detecting comprises a wavelength-scanning optical monitor.
The invention also provides optical communication apparatus comprising: an optical transmitter; a dispersive optical fibre link; and an optical device as defined above, the device having a dispersion characteristic acting against the dispersion of the optical fibre link.
Preferably the communication apparatus comprises an optical receiver for receiving optical signals transmitted via the optical fibre link;
and the means for detecting comprises means for deriving an electrical signal indicative of the magnitude of the output of the optical receiver.